The present invention relates to an improved boiler wherein a combustion gas duct is divided into a plurality of passages, the separate passages being provided with reheaters and superheaters, and the amount of combustion gas which flows through the passages to the discharge end thereof is adjustable by means of dampers provided at the outlet ends of the passages.
In prior art boilers with reheaters incorporated therein, known as reheat boilers, a damper control system has been utilized as one system for controlling reheat steam temperatures. In this system, the gas duct is divided into a plurality of passages. The separate passages are provided with reheaters and superheaters. In addition, a damper is provided at the outlet end of each passage. With this arrangement, the amount of combustion gas flowing through each passage is adjusted by controlling the opening of the dampers, thereby controlling the heat absorption by the reheater, and thus controlling the reheat steam temperatures.
During a the rapid load down or in case actual operating conditions are different from planned operating conditions, reheaters must absorb a greater amount of heat than planned. In such a case, the opening of the dampers is so controlled as to direct more combustion gas into the passages with the reheaters provided therein. On the other hand, the amount of combustion gas flowing through the passages with the superheaters provided therein is naturally decreased. As a result, the heat absorption by the superheaters decreases control of steam temperatures at the outlet of each of the superheaters which is effected by a fuel-feed water ratio control system. Accordingly, as the heat absorption by the superheaters decreases, more fuel is supplied to the furnace for compensating purposes. However, in the event that a temperature exceeds certain tolerance limits, the water-wall tubes constituting the furnace are subject to thermal strain due to large temperature differences thereacross. A problem arises of protecting the furnace.